Android: kill all adb instances before trying to restart usb.

On some bots, there are adb instances left over from previous runs.
They don't play well with restarting the usb, and devices eventually "disappear".
This tries to kill all adb instances before restarting usb.

BUG=311058
TBR=navabi@chromium.org

Review URL: https://codereview.chromium.org/39833003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231035 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231035